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The present invention relates to vitreous bond grinding elements containing metal coated superabrasive particles or grit and more particularly to the use of functionally graded coatings for improved wetting of the coated particles by the vitreous bond matrices.
Refractory metal oxides (titania, zirconia, alumina, silica) coatings on diamond, CBN, and silicon carbides for use in grinding elements have been proposed (see U.S. Pat. Nos. 4,951,427 and 5,104,422). Processing of these coatings includes deposition of an elemental metal (Ti, Zr, Al) on the abrasive particle surface followed by oxidizing the sample at an appropriate temperature to convert the metal to an oxide. Unfortunately, these coatings often crack do to inherent brittleness of the oxide layer and residual stresses developed from the mismatch in thermal expansion coefficients between the oxide and abrasive particle. In practice, these abrasive particles pull out of the matrix during grinding when the interface between the coating and abrasive particle is weak.
Retaining the crystals in the bond is a major factor that determines the usable lifetime of a grinding wheel. Coated crystals pulling out from the vitreous bond during grinding is one of the major failure modes of grinding wheels. There are two bond lines to consider when working with coated abrasive crystals. One bond line is the diamond/coating bond line and the second is the coating/matrix bond line. Failure of either bond will result in pull out of the coated crystals and shortened life of the grinding element containing the coated crystals. Thus, there exists a need in the art for coated abrasive crystals, which provide improved retention in vitreous bonds.
The retention of metal oxide coated superabrasive particles in a vitreous bond matrix is improved by incorporating functionally graded coated abrasive particles in the vitreous bond matrix. Diamond, cubic boron nitride, silicon carbide, and like abrasive particles are useful for this purpose. The novel functionally graded coated abrasive particles have an outer oxide phase coating layer derived from a metal carbide, nitride, and/or boride, which forms an inner coating layer. The coating provides improved adhesion of the abrasive particles in the matrix and protection against chemical attack during tool fabrication and processing.
The invention, then, is directed to coated abrasive particles for use in vitreous bond matrices. The particles are coated with between 1 and about 50 coating layers. Each coating layer ranges in thickness from between about 0.1 and 50 microns. Each layer has the composition, MCXNYBZOW, where, M represents one or more of Ti, Si, V, Cr, Zr, Nb, Mo, Hf, Ta, W, Re or a rare earth metal, and w, x, y, and z, each range from between 0 and 3. The outermost coating layer has a concentration of oxygen that is higher by a factor of at least about 2 than the layer in direct contact with the abrasive particle.
The coating layers can be formed by depositing between 1 and about 50 layers of a coating of the composition, MCXNYBZ. An outermost layer(s) enriched in oxygen by a factor of at least about 2 compared to the coating layer in direct contact with the abrasive particle is formed by reaction of oxygen with the carbide/nitride/boride coated abrasive particles.
The functionally graded coated abrasive particles, then, can be incorporated with matrix materials for forming a vitreous bond grinding element. Matrix materials include vitreous bond material, SiO2, B2O3, Na2O, CaO, MgO, or other similar glass forming materials. Conventional processing common in the art is used to form the vitreous bond grinding elements. Improved grinding performance results from using such grinding elements